How many bits of information make a lifetime? Not the number of raw bits of imagery that a pair of human eyes can take in (a huge but uninteresting value), but rather the sensible and meaningful bits that can be the subject of conscious thought. How many bits, in other words, does a mind get to work with?

A good answer for the "bit rate" of cognition is of the order of speech or internal narration rates, as suggested by Daniel Dennett's model of mind as the "narrative center of gravity" (in Consciousness Explained). That's arguably how we really think. A rough but reasonable upper-bound value for that is ~1 kb/s --- about 100 bytes per second. And that may be rather generous; 1% to 10% of that could be more likely, most of the time. Multiply that by a few decades of active waking life, roughly a billion seconds, and the output is ~100 GB, a tenth of a Terabyte.

That's big, but not impossibly huge to store inside a cranium. A CD-ROM holds several hundred MB, so our estimate comes to a thousand or so CDs. But note that the data of life may compress rather well --- most memories are of sequences of events, reasonably continuous and predictable --- so nobody is suggesting that a lifetime fits onto a bookshelf of music albums!

But it is more than a little humbling to think that all the events of a life, or at least all the events that one can possibly remember and think about, are expressible in under a trillion bits....